api: prevent evaluated derivatives to be re-evaluted

Author: mloubout

devito/types/basic.py

@@ -1011,7 +1011,7 @@ def c0(self):
     def _eval_deriv(self):
         return self
 
-    @property
+    @cached_property
     def _grid_map(self):
         """
         Mapper of off-grid interpolation points indices for each dimension.